Exposure, heat development and fixing of photosensitive mercurous oxalate and silvercompositions



United States Patent No Drawing. Filed Apr. 5, 1962, Ser. No. 185,191 8 Claims. (Cl. 96-27) This invention relates to an improved data-processing method and more particularly to a method for enhancing the responsivity of photothermographic compositions, particularly those containing mercurous or silver oxalate to light exposure.

Photothermographic compositions are well known. An early disclosure of these compositions is seen in US. Patent No. 1,976,302, which issued in 1934. This patent covered the use of mercurous and silver oxalates for making photographic imagesi The patent explained the mechanism as an initial inoculation of the substance with nuclei of unspecified composition, by exposure to light. Upon application of heat, these nuclei subsequently catalyze the thermal decomposition of the oxalate to develop the latent image. Unfortunately, the period needed for develop: ment, as well as the contrast obtained, left much to be desired.

Since the issuance of that patent, much eifort has been expended directed to enhancement of the responsivity of photothermographic compositions to exposure and development. Only moderate success has been attained.

Accordingly, an object is to provide a novel photographic method, which is fast and economical in operation.

Another object of this invention is to provide a novel method for enhancing the responsivity of photothermographic compositions to light exposure.

Another object is to provide a method for making photothermographic compositions more sensitive to light in any desired range of the spectrum.

A further object is to provide a method for increasing the photolytic rate of photothermographic compositions.

Another object is to provide a novel photographic exposure developing and fixing process for photothermograph ic compositions wherein the photographic image will be made permanent and possess archival qualities.

Other objects of the invention will in part be obvious and will in part appear hereinafter.

The invention accordingly comprises the several steps and the relation of oneor more of such steps with respect to each of the others, which are exemplified in the following detailed disclosure, and the scope of the invention will be indicated in the claims.

In general, we have found that when photothermographic compositions are heated simultaneously with their exposure to light, a synergistic enhancement of the photolytic rate occurs. A consequent reduction in development time due to an increased photolytic rate can therefore be made. More particularly when the photothermographic composition is kept heated at a temperature ranging from 125' F. up to a temperature below that at which the photothermographic composition will decompose, during exposure to light, the rate of darkening during light exposure is substantially increased and less energy is subsequently needed to develop the composition. Indeed, the time needed for exposure and development is reduced to such an extent that use of these compositions in commercial photography becomes quite practical.

With respect to the mode of operation, the photothermographic compositions applicable herein are ordinarily used along with an anhydrous or a gelatinous medium. The mixture is coated onto a substrate, such as a t excess).

3,259,494. Patented July 5, 1965 ice cellulose acetate sheet, paper, cloth or a glass plate, and

dried before use. It is then exposed to light while in a heated condition and developed with heat.

Certain methods of stabilization or fixing of the image use, so that upon development, the ferrous ion will be converted to the ferric ion. The latter'inhibits further development of the oxalate composition and therefore the image is stabilized. A combination of the two techniques is also possible.

The coated plate or sheet, with or without the stabilization additives, is then briefly exposed to a source of actinic energy, the exact time depending on the intensity. If a dye has been incorporated into the photothermographic composition, certain wave lengths of actinic energy may be more .desirable than others. coated substrate is kept in a heated condition, either by simultaneous application of heat or by residual heat. .The temperature in the heated condition should range upward of F. for the best results.

After exposure, the latent image on the sheet is developed by exposure to a temperature between about 180 to 350 F. For example, an oven, a heated roll, a heated stylus or an infra-red lamp may be used for this purpose.

If the pre-stabilization techniques above are not used or if it is desired to supplement these methods, certain other fixing methods can now be utilized to stablize the developed image. More specifically, unreacted photothermographic composition may be removed by transfer or washing or the reacted portions may be removed by transfer or volatilization. Each method is described in detail below.

Upon completion of the process, it will be found that an image of good density is produced even though a shorter period of exposure to light and to heat'is utilized.

In essence, the light energy required has reduced almost comes a practicality.

The invention will now be described in detail by reference to specific examples which disclose the manner of exposure and the techniques for developing the coated substrate to produce a visible image.

Example I A solution of 4.8 parts 'by'wei-ght of HgNO .H O and 4 parts BaNO .2H O in 90.2 parts of 0.3 N nitric acid was treated with 0.7 part of urea and added rapidly in the dark to 124 parts of 0.36 N potassium oxalate (25% The mixture was then stirred for 10 minutes, after which it was passed through a homogenizer three times. The precipitate was allowed to settle and washed by decantation with distilled water until the wash was oxalate free.

The washed precipitate was then allowed to contact 50 parts of 2.0 N potassium oxalate for 2 hours, and subsequently it was dehydrated by 4 washings with acetone. Next it was added to a solution of 55 parts of half-second cellulose acetate-butyrate in parts of acetone contain ing 16.5 parts of dioctyl phthalate as a plasticizer and then emulsified in a homogenizer. The resultant emulsion was used to coat a roll of paper. The paper was then exposed During exposure, the

' nizer.

was equivalent to a paper speed of 5000"/minute at a spot-speed of 3000 cycles/second. Simultaneously, dun ing exposure to the spot of light, the paper was heated to 170 F. with an infra-red lamp facing its uncoated side. Following this exposure to light and heat, the paper was developed by a 4 second passage over a heated roller maintaincd at 300 F. The resultant image had unusually good optical density. When compared with similarlyconstituted photographic paper which had not been heated during exposure to light, a noticeable increase in optical density appeared even though a lesser development time had been used.

Example II A solution of 9.6 parts of HgNO J-I O in 90.2 parts of 0.3 N nitric acid was treated with 0.7 part of urea and added rapidly to 124 parts of 0.36 N potassium oxalate (25% excess) in the dark andstirred for 10 minutes. The mixture was passed through a homogenizer three times. The precipitate was allowed to settle and washed by decantation with distilled water until the wash was oxalate free. The volume of the mcrcurous oxalate and supernatant liquid was then adjusted to the original total volume. To this suspension was added 225 parts of a warm solution of gelatin (10% by weight) of iso-electric point less than 7 and 19 parts of 2 N potassium oxalate. The suspension was emulsified by two passes through a homoge- The resultant emulsion was used to coat a number of glass plates.

After drying, each plate was heated for 10 seconds at 248 P. Then, within minutes and while the plate was still hot, it was placed against a printed translucent sheet and exposed to a 4 watt mercury fluorescent light in a direct process for one second at a distance of 1 foot from th lamp. Each plate was then developed for 4 seconds, using the heat in an oven kept at 300 F. The developed plates were then compared to others which, although manufactured in the same way and with a similar photothermographic composition, were exposed to the same source of light but under varying temperature conditions. The following table sets forth the results:

1 Plates exposed live minutes after heating for seconds at 248 F.

' Plates exposed without application of heat.

3 llatcs exposed after 4 hours residency in a dry atmosphere preceded by a 10 second ex osurc to heat at 248 F.

4 Al) =Optical cusity of image less the optical density of background.

I Al).=retlectance ditfercucc.

Example III A mixed mercurous-silver precipitate was made in accordance with the above techniques. This involved forming a solution, in the dark, of 2.5 parts of HgNO .H O, 1.5 parts of AgNO 3.9 parts of BaNO 2H 0 and 92 parts of 0.3 N nitric acid. This solution was then added to a excess of0.36 N potassium oxalate and homogenized. The precipitate was removed, washed with water or acetone, and admixed with gelatin or butyrate as above, and then coated onto cellulose acetate sheets. After being dried, each cellulose sheet was positioned against a printed paper sheet, in a reflex arrangement, and exposed to one flash of about 1-4 microsecond duration of a 40 wattsecond flash lamp disposed at a distance of 3 feet while the sheets were heated at 125 F. Each sheet was then developed for seconds by heat from an oven maintained at 200 F.. An itnagc of unusually good resolution and contrast was produced in each case.

Example IV A solution of 9.6 parts of HgNO .H O in 90.2 parts of 0.3 N nitric acid was treated with 0.7 part of urea and added rapidly to 124 parts of 0.36 N potassium oxalate (25% excess) in the dark and stirred for 10 minutes. The mixture was passed through a homogenizer three times. The precipitate was allowed to settle and washed by dccantation with distilled water until the wash was oxalate frcc. Prior to the final washing, 0.003 part of Acridine orange was added to the wash liquid for absorption by the precipitate. The dye shifted the wave length sensitization range of the photothermographic composition.

Th volume of the mercurous oxalate and supernatant liquid was then adjusted to the original total volume. To this suspension was added 225 parts of a warm solution of gelatin (10% by weight) of iso-electric point less than 7 and 19 parts of 2 N potassium oxalate. The suspension was emulsified by two passes through a homogenizer. The resultant emulsion was used to coat a number of glass plates.

After drying, each plate was heated for 10 seconds at 248 F. and then within 5 minutes and while the plate was still hot, (about 158 F.) placed against a printed translucent sheet and exposed to a 4 watt mercury fluorescent light in a direct process for one second at a distance of 1 foot from the lamp. Each plate was then developed using heat from an oven kept at 300 F. for 4 seconds. A developed image was obtained having good resolution.

To effect stabilization of the developed image, the following techniques were employed:

Example V The coated roll of paper of Example I was additionally coated prior to exposure with a 0.5 to 10 mil thickness coating of polyvinyl butyral. This added coating was exposed to heat and became less transparent to ultraviolet light. It thereafter acted as a screen to subsequently block out UV rays so that the image was stabilized.

After exposure and development, and upon test, it was found that the material so treated had a photolytic rate of 1.5 x 10- optical density units per second. In contrast, untreated material had a photolytic rate of 5 X 10* optical density units per second.

The photolytic rate of the treated material corresponds to a stability of 20 hours in a carbon are such as found in an Atlas Fadeometer. It is evident that image stability was attained.

Example VI To a portion of the emulsion prepared as in Example 11, there was added FeC O .2H O in a molar amount in excess of the soluble oxalate in the emulsion (3 parts, in this instance) prior to use as a coating composition for the substrate.

By oxidation during the heating, the ferrous ion was converted to ferric ion which inhibited subsequent thermal amplification of the photothermographic composition. Since thermal amplification was inhibited, considerable stabilization was achieved.

Example VII To a second portion of the emulsion prepared as in Example II, three parts of FeC O .2H O were added. Glass plates were then coated with the modified emulsion, and after being dried, a coating of two mil thickness of polyvinyl butyral was applied to the coat surface.

When used subsequently as photographic material and then developed, the plates were found to possess excellent image stability.

Example VIII This stabilization technique is a post-exposure and development step wherein the reacted photocomposition is transferred to a second sheet to form a negative which is stable.

After the image was formed by exposure and development, the mercury of the image was caused to react with either selenium sulfide or palladium chloride which was contained on a transfer sheet. The latter was pressed against the photographic sheet to cause a transfer. Whereever there was mercury on the original, a blackening of the transfer sheet occurred.

Example IX In another fixing technique, prolonged heating (about 30 minutes) of the paper roll as prepared in Example I, produced a direct positive by reason of a faster rate of volatilization of themercury from the image areas. Mercury was left behind in the non-light struck areas due to a rate phenomenon. A positive image resulted. The image was then stabilized by the transfer technique of Example VIII.

Example X Using still another stabilization method, some of the exposed and developed glass plates of Example IV, were dipped in a 0.1% to saturated solution of NaCl (in this instance-a solution) for seconds to 2 hours (in this instance2 minutes) and then washed with water and dried. This effected conversion of the unreacted oxalates to their chlorides. The image then had good stability, being legible for at least-40 hours in the Fadeometer.

Example XI In this stabilization method, used on a number of the developed plates made as in Example IV, unreacted Hg C O was removed by first hardening the gelatin with a formaldehyde solution for one minute, washing with water and then dipping into either a 65% perchloric acid or a saturated zinc nitrate solution for 2-5 minutes. The substrate was then washed andv dried. Since the unreacted oxalates had been removed, a fixed permanent image was produced.

As illustrated by Example IV, the wave length sensitization of the composition can be altered by inclusion of dyes. Other dyes besides Acridine orange that are useful are found in Photographic Engineering, volume 4, No. 2 (1953), and in treatises on photographic techniques.

Variations in the process are possible. The method of exposure, as illustrated by the examples, may be either direct, indirect or reflex. The range of temperatures employed during light exposure may vary from 125 F. up to a temperature below that at which the photothermographic composition will decompose. For heat development, the preferable temperatures are from about 180 F. to 350 F. and more preferably from 200 F. to 300 F. Temperatures outside of this range are operable but require much closer control at the higher temperatures and unduly long periods of development at the lower temperatures.

Another modification of the present invention would be to expose the material to light in selected areas and then heat in selected areas. The photothermographic medium could be uniformly exposed to light and then heated in selected areas to record data. Light and temperatures within the ranges described in the examples are useful. A further modification is the use of photothermographic composition as described in the preceding examples and heat to substantially 150 P. Then expose the material to a lamp that forms a latent image over the entire surface uniformly, e.g., a flash lamp at a distance of two feet. The sheet is then placed over the material to be copied and exposed to infrared radiation, for example, from a thermographic copying machine. The radiation causes the image areas on the original to heat. The heat is transferred to the photothermographie sheet causing development of parts of the uniform-latent image to produce a copy. Since the sheet containing the latent image darkens wherever sufiicient heat is applied, it will be apparent that other means of applying the heat, e.g., heated stylus, projected infared radiation modified by the data, may be used without departing from the invention.

It will thus be seen that the objects set forth above, among those made apparent from the preceding description, are efliciently attained, and since certain changes may be made in carrying out .the above process without departing from the scope of the invention, it is intended that all matter contained in the above description shall be interpreted as illustrative and not in a limiting sense.

Particularly, it is to be understood that in the claims, ingredients or compounds recited in the singular are intended to include compatible mixtures of such ingredients wherever the sense permits.

Having described our invention, what we claim as new and desire to secure by Letters Patent is:

1. In a process for recording data in a photothermographic medium which comprises a thermally responsive image forming system comprising a substance chosen from the group consisting of mcrcurous oxalate, silver oxalate and mixtures thereof, said process including the steps of;

(a) forming a latent image in said medium by exposure of said medium to a source of light modified by the data to be recorded therein, and

(b) developing said image by heat, the improvement which comprises, maintaining said medium at an elevated temperature which is below that at which development occurs simultaneous with said expo-.

sure of said medium to said light source.

2. The process of claim 1 including the additional step of heating said medium to a temperature below which development occurs prior to said exposure of said medium.

3. The process of claim 1 in which during said expo-.

4. The process of claim 1 including the further step of fixing the developed image to stabilize said image.

5. A photothermographic process for recording data in a photothermographic medium which comprises a thermally responsive image forming system comprising a substance chosen from the group consisting of mercurous oxalate, silver oxalate and mixtures thereof which comprises the steps of v (a) heating said medium at a temperature between about F. to a temperature below which development occurs prior to said exposure of said medium,

(b) forming a latent image in said medium by exposing said medium to a source of light modified by the data to be recorded therein,

(c) simultaneously maintaining said medium during exposure thereof at a temperature ranging from between 125 F. to a temperature below which development occurs and,

(d) developing said image by heating said exposed medium to a temperature between about F. to about 350 F.

6. The process of claim 5 including the further step of fixing the developed image to stabilize said image.

7. The process of claim 1 wherein said thermally responsive system comprises mercurous oxalate.

8. The process of claim 1 wherein said thermally -responsive system comprises silver oxalate.

References Cited by the Examiner NORMAN G. TORCHIN, Primary Examiner.

A. D. RICCI, Assistant Examiner. 

1. IN A PROCESS FOR RECORDING DATA IN A PHOTOTHERMOGRAPHIC MEDIUM WHICH CCOMPRISES A THERMALLY RESPONSIVE IMAGE FORMING SYSTEM COMPRISING A SUBSTANCE CHOSEN FROM THE GROUP CONSISTING OF MERCUROUS OXALATE, SILVER OXALATE AND MIXTURES THEREOF, SAID PROCESS INCLUDING THE STEPS OF; (A) FORMING A LATENT IMAGE IN SAID MEDIUM BY EXPOSURE OF SAID MEDIUM TO A SOURCE OF LIGHT MODIFIED BY THE DATA TO BE RECORDED THEREIN, AND (B) DEVELOPING SAID IMAGE BY HEAT, THE IMPROVEMENT WHICH COMPRISES, MAINTAINING SAID MEDIUM AT AN ELEVATED TEMPERATURE WHICH IS BELOW THAT AT WHICH DEVELOPMENT OCCURS SIMULTANEOUS WITH SAID EXPOSURE OF SAID MEDIUM TO SAID LIGHT SOURCE.
 4. THE PROCESS OF CLAIM 1 INCLUDING THE FURRTHER STEP OF FIXING THE DEVELOPED IMAGE TO STABILIZED SAID IMATE. 